Conventionally, a vehicle drive control device automatically drives a vehicle by executing, for example, a cruise control, by which the vehicle is driven so as to maintain a set vehicle speed without an acceleration operation conducted by a driver, or an adaptive cruise control (ACC), by which the vehicle is driven so as to maintain a predetermined distance from a vehicle traveling in front of the subject vehicle (hereinafter referred to as a leading vehicle) without the acceleration operation or a brake operation conducted by the driver.
For example, in a case where the vehicle travels a road surface having a low coefficient of friction (hereinafter referred to as a friction coefficient), a driving torque, a braking torque and the like for automatically driving the vehicle may become excessive, thereby causing a slip at each wheel of the vehicle. As a result, the conventional vehicle drive control device has disadvantages such that a driving state of the vehicle may become unstable, and the automatic drive of the vehicle may be forced to be cancelled due to an occurrence of the slip. In order to eliminate the drawback mentioned above, a vehicle drive control device disclosed in JP2004-17731A automatically drives a vehicle while preventing the occurrence of the slip when a road surface having a low friction coefficient exists in a prospective traveling route by calculating a limit vehicle speed, by which the occurrence of the slip is prevented, on the basis of the friction coefficient of the road, surface, and then decelerating the vehicle speed to the limit vehicle speed before the vehicle reaches the subject road surface.
Whether or not a slip occurs at a wheel is determined on the basis of a relationship between a friction coefficient of a road surface and a torque acted on the wheel. Therefore, the relationship between the friction coefficient of the road surface and the torque acting on the wheel is not taken into consideration in the case of the vehicle drive control device disclosed in JP2004-17731A that only restrains the vehicle speed. Hence, the vehicle drive control device disclosed in JP2004-17731A may not appropriately and surely restrain the occurrence of the slip.
A need thus exists for a vehicle drive control device which is not susceptible to the drawback mentioned above.